20080619

The Reality TestsVienna In Seed (June 2008), the Vienna experimental group discusses physical and philosophical implications of new correlations between entangled photons, which violate an inequality proposed by Leggett for nonlocal realistic theories. This new series of experiments invalidates macrorealism in quantum mechanics by more than 80 orders of magnitude. Preliminary coverage of the experimental results was first presented in Nature446 (871) and PhysicsWorld, 20 April 2007. According to Časlav Brukner: "Quantum mechanics does not always wash itself out – but to observe its effects for larger and larger objects, we would need more and more accurate measurement devices. We just do not have the sensitivity to observe the quantum effects around us. In essence, we do create the classical world we perceive. There could be other classical worlds completely different from ours."

Quantum networks: Entanglement of distant atoms by projective measurementUniversity of Barcelona | ICFO | Spain Quantum cryptography is rapidly developing into a mature and robust technology for secure data transactions in financial, government and military sector applications. In arXiv 0806.1052, Zippilliet al. quantify the role of photon detector efficiency in quantum repeaters, which will be necessary to scale beyond the point-to-point networks currently employed for secure communications.

Presently, state-of-the-art systems employ atom-photon interaction to generate entanglement between distant nodes across a quantum network through projective measurement. "We assess proposals for entangling two distant atoms by measurement of emitted photons, analyzing how their performance depends on the photon detection efficiency – we believe that these concepts are generally applicable to all systems that may be considered for the creation of distant entanglement, including atomic-ensemble, photonic, and solid state implementations."

The groups's objectives are to quantify the importance of detector efficiency as applied to generating remote entanglement across quantum networks. With minor modifications, these results can be extended to the efficiency of quantum teleportation protocols that are also based on projective quantum measurement. "In all such systems, the detection efficiency will have a similar, important role for the use of the entanglement as a resource in quantum technologies."

Space-QUEST: Experiments with quantum entanglement in spaceVienna | ESA | ISS In a recent submission to the arXiv, Zeilinger's group at University of Vienna, Austria has proposed an experiment – Space-QUEST, Quantum Entanglement Science and Technology – for space-to-ground, entangled-photon Bell Inequality violation measurements to verify quantum nonlocality at distances over thousands of kilometers, in a joint operation between the International Space Station and a ground observatory in the European Union.

Entanglement and nonlocality have been pivotal controversies since the birth of quantum mechanics – Einstein's "spooky action at a distance" implies simultaneous, nonlocal correlations between separate entangled particles. J. S. Bell was the first to confirm the phenomenon experimentally in 1964.

Further refinements and increasing precision in succeeding experiments have consistently shown quantum mechanics to be an explicitly nonlocal theory – the outcome Einstein was most averse to accept. However, long-distance relativistic experiments, such as between orbiting satellites, have been technologically cost-prohibitive to date. The paper will be presented at the 2008 IAC Microgravity Sciences and Processes Symposium, under a proposed joint initiative between the European Space Agency and the International Space Station.

"Testing quantum correlations over distances achievable with systems placed in the Earth orbit, or even beyond, would allow to verify both the validity of quantum physics and the preservation of entanglement over distances impossible to achieve on the ground. Using the large relative velocity of two orbiting satellites, one can perform experiments on entanglement where – due to special relativity – both observers can claim that they have performed the measurement on their system prior to the measurement of the other observer. In such an experiment, it is not possible anymore to think of any local realistic mechanisms that potentially influence one measurement outcome according to the other one."

Zeilinger's group has previously conducted proof-of-principle experiments in the Canary Islands with a 144 km free-space link, using an ESA receiver telescope to receive single entangled photons [Nature Physics, 3:481-486 (2007)]. A more recent experiment in Italy has demonstrated single-photon downlink communications viability from a near-earth orbit satellite [New Journal of Physics, 10:033038 (2008)].